1. Field of the Invention
The invention concerns a process for excavating by dissolution, an underground cavity in a thin salt layer, for example an underground salt layer.
More particularly, the object of the invention is to obtain, after excavation, a cavity for the storage of a fluid, in particular natural gas, in a salt cavern obtained after the dissolution process.
2. Description of the Background Art
In view of the stresses to which the cavity has to be subjected during its use as an underground gas storage, the dissolution process has to be controlled in order to ensure that the final cavity has a mechanically stable shape.
Although it is relatively easy to control dissolution when the thickness of the available salt is several hundred meters thick or so, this operation is more awkward when the salt is stratified and of more reduced thickness. In fact, even when the thickness of the salt available is only a few hundred metres thick, it becomes necessary to apply specific cavern dissolution processes to develop cavities whose width over height ratio is of the order of 1. When the salt thickness is less than about 100 metres, new salt dissolution processes are needed. The invention concerns a process for developing a tunnel-shaped cavern in such a th in salt layer.
Certainly, U.S. Pat. No. 5,246,273 discloses for the excavation of a salt layer by dissolution:
producing an injection duct, an extraction duct, a void for a communication space which places the injection and extraction ducts in communication, and at least one void for a blind tunnel such that: PA1 then injecting via the injection duct a salt solvent into the communication space in order to excavate the cavity by dissolution of the salt on contact with the solvent; and PA1 then extracting via the extraction duct the brine formed by the dissolution of the salt. PA1 forming the open end of the blind tunnel void in the vicinity of the injection duct; PA1 circulating the solvent in the injection duct and making it emerge from this duct via an end which forms the point of injection into the cavity; and PA1 forming the blind tunnel void such that the part with its open end overhangs the injection point. PA1 the injection and extraction ducts are produced at a spacing from each other; PA1 an elongate part is provided in the communication space void; and PA1 the void for the blind tunnel and the elongate part of the communication space void are produced substantially as an extension of each other, and the injection duct is produced between the blind tunnel void and the elongate part of the communication space.
the blind tunnel extends between an open end and a closed end; and PA2 the blind tunnel communicates via its open end with the communication space;
In this document, U.S. Pat. No. 5,246,273, the communication space and the blind tunnel are excavated as an extension of each other, sloping downwards from the injection shaft such that the closed end of the injection shaft is lower than its open end. The blind tunnel is an optional sump designed to recover the insoluble elements deposited therein.
Consequently, the technique applied in U.S. Pat. No. 5,246,273 is costly since, related to the volume of the final cavity with a mechanically stable shape, the number of production operations is relatively high (in particular producing the ducts and the void for the communication space).
In order to overcome these problems, the invention proposes that, to excavate the cavity, the blind tunnel also be excavated by circulating the solvent in this tunnel, making the solvent pass into the tunnel via its open end from the communication space and recovering the resultant brine so that it can be extracted therefrom.